1. Field of the Invention
The present invention relates to an illuminating system for generating a high-intensity illuminating light utilizing a lamp of the short arc type, and more particularly to a cooling structure for such illuminating system.
2. Description of the Prior Art
Mercury lamps of the short arc type are often employed in apparatus for exposing a photosensitive material to high-intensity light. Such lamp, generally capable of emitting light of a determined wavelength with a high efficiency, is suitable as a light source of an exposure apparatus for transferring the pattern of a photomask onto a semiconductor wafer. However such arc lamp consumes a large electric power, so that the discharge electrode, lamp wall housing the discharge electrode and metal mount of the lamp often reach a high temperature in the order of 200.degree. to 500.degree. C. particularly in case a spheroidal reflector is employed for efficiently concentrating the light from the arc lamp, the temperature rise becomes even larger and may lead to the destruction of the arc lamp unless suitable cooling system is provided, since the spheroidal reflector is positioned to surround the lamp. For achieving such cooling, the Japanese Patent Publication No. 43226/1980 discloses a method of blowing cooling air from a nozzle to the metal mount of the lamp and to so intercept the cooling air as not to reach the lamp wall. However, if the average input electric power to the arc lamp fluctuates, the metal mount and the lamp wall show considerable variation in temperature, and, in the above-described method, it becomes necessary to blow the cooling air of a corresponding amount from the nozzle. Although it is possible to automatically control the amount of air supplied from the nozzle by detecting the temperature of the metal mount and the lamp wall, a large amount of air is required if the temperature of the metal mount rises, and the apparatus becomes inevitably large because of the requirement for a high-capacity source for compressed air such as a compressor. Experiments conducted by the present inventors have revealed that a flow rate of several liters per minute is required to satisfactorily cool the lamp wall or metal mount in case the temperature thereof reaches 100.degree. C. or so. Dusts of several microns, sometimes those as small as one micron cannot be tolerated in the exposure apparatus for semiconductor device manufacture. However the use of a large amount of cooling air as in the above-described prior art often gives rise to the involvement of dusts of untolerable size, and is therefore undesirable for the exposure apparatus in the field of semiconductor device manufacture. Besides, as the nozzle is directed to the metal mount of the lamp, this method is effective for cooling the arc lamp alone but is not effective for cooling the entire illuminating system including the spheroidal reflector.